Chronic myelogenous leukemia is characterized by a reciprocal translocation between chromosomes 9 and 22 of man. This results in an abbreviated chromosome 22 termed the Philadelphia chromosome which is found in over 95 percent of patients with chronic myelogenous leukemia and in a minority of patients with acute lymphoblastic leukemia. In this translocation, the abl gene from chromosome 9 is spliced to a region on chromosome 22 called the breakpoint cluster region (bcr). This fusion gene, bcr-abl, is transcribed into a unique RNA transcript that is about 8 kb in size.